Re: Continuations with pictures

[P T Withington <address@hidden>]

Or _Groundhog_Day_.

On Friday, Aug 8, 2003, at 14:01 US/Eastern, Rob Hunter wrote:

> See the movie _Memento_!
>
> Not only is a great film, but, apparently, it will teach you about 
> continuations as well.
>
> Rob
>
> On Friday, August 8, 2003, at 11:05 AM, Kimberley Burchett wrote:
>
>>
>> On Fri, 8 Aug 2003, Kevin Kelleher wrote:
>>
>>> So here is a picture -- maybe you can tell me what's wrong
>>> with it:
>>
>> It seems like you're thinking of all the junk in the room as being 
>> part of
>> the continuation.  But it would probably be more appropriate to think 
>> of
>> the junk as global variables, that would still be wherever they were 
>> even
>> after you invoked the continuation...
>>
>> I think it would be easier to offer a slightly different picture 
>> rather
>> than try to fix up bit by bit the one that you illustrated.
>>
>> A continuation is more like a little envelope containing instructions
>> about what to do next.
>>
>> Imagine that right after your daughter cleaned up her room, she 
>> wanted to
>> go get some ice cream as a reward.  She knows that she's prone to 
>> forget
>> these things so she decides to write a note to remind herself (yes, 
>> it's
>> very odd for a child to not only forget the reward, but also have the
>> foresight to write it down, but bear with me).  On the outside of the
>> envelope, she writes "Do this next".
>>
>> Now her friends come over, and her room gets messed up again.  Time
>> passes...
>>
>> The next morning, she wakes up and in the midst of the mess, she 
>> notices
>> an envelope labeled "Do this next".  She opens it, and it tells her 
>> to go
>> get ice cream.  She thinks this sounds like a good idea, so she does.
>>
>> Notice that she didn't put the room back in order first.  The room is 
>> all
>> the global variables -- nothing happens to them simply by invoking the
>> continuation.  Basically, you can think of the continuation as a 
>> "train of
>> thought".  When she wrote down the note (created the continuation), 
>> she
>> had intended to get ice cream.  She then promptly forgot this plan, so
>> that train of thought disappeared.  But then she read the note 
>> (invoked
>> the continuation) and the train of thought was resumed.  Notice also 
>> that
>> the original motivation for getting ice cream was as a reward for 
>> having
>> cleaned up the room -- but the fact that the room is now messed up 
>> again
>> doesn't prevent her from getting ice cream anyway.
>>
>> The label on the envelope is arbitrary -- it's simply the name of the
>> variable that holds the continuation.  You could label it anything you
>> wanted, but it would still have the effect of changing your train of
>> thought.
>>
>> There's no such thing as a future continuation, since nobody has yet 
>> had
>> that train of thought, so they can't write it down.  There is a 
>> difference
>> between upward and downward continuations, but the difference is 
>> mostly a
>> matter of implementation and how you use them.
>>
>> I didn't address all the issues involved in continuations -- e.g. 
>> where
>> they "return to", but I think this conveys the gist of the concept at 
>> the
>> same level that you were originally asking about.
>>
>> --
>> Kimberley Burchett
>> http://www.kimbly.com/
>
>